Study on kinetics of electro-optical and photoluminescent processes in nanostructured transition metal (W, Mo) oxide-based thin films

1.4.4 Thin film batteries [17, 33] The similar features of electrochromic devices and rechargeable thin film batteries concern the material properties, structure of the layers, kinetics

Lê Hà Chi

Nghiên cứu Động học các quá trình biến đổi điện - quang - quang tử của màng mỏng vật liệu ôxít kim loại chuyển tiếp (W, Mo) cấu trúc nanô

Chapter 1 Overview of transition metal (W, Mo) oxides and their electrochromic

properties 2

1.1 Introduction of transition metal 2

1.2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4] 4

1.3 Properties of tungsten oxide and molybdenum oxide 7

1.4 Applications for electrochromic materials 11

Chapter 2 Photoluminescent properties of nanocomposite materials 20

2.2 Fluorescence and phosphorescence (photoluminescence) [20] 20

2.2 Physics of nanostructured materials [7] 23

2.3 Enhance photoluminescent performance of nano-composite materials 33

Chapter 3 Experiments 39

3.2 Preparation by electrochemical method 39

3.2 Preparation by thermal oxidation method 45

3.3 Study on morphology and structure of the films 48

Chapter 4 Kinetics of electro-optical transformation processes of nanostructured WO 3 -based thin film 55

4.1 Ion intercalation/extraction studied by electrochemical techniques 55 4.2 Electro-optical properties of WO 3 -based electrochromic device studied in-situ

5.4 I-V characteristics studied by electrochemical technique 79 Conclusion 83 References 85

Preface

The purpose of this work is to prepare nanostructured transition metal (W, Mo) oxide - based thin films and study their kinetics of electro-optical and photoluminesent processes It is known that electrochromic materials have been found many interests with the respect not only to the fundamental studies, but also to the application scopes, such as solar energy management, sensors and display devices [1,3] Among these electrochromic materials, tungsten oxide films are by far the most extensively studied

WO3 is a wide band gap semiconductor with Eg ≈ 3.2 eV, it thus transparent in the visible light range [3]

Electrochromic tungsten oxide films can be prepared by a variety of different techniques such as physical vapor [2] and chemical vapor deposition [14,29], electrochemical deposition [13,34], sol - gel [25], etc The electrochemical deposition

is expected to be one of the most economical methods for making a large-area film as well as automatically controlling the film growth However, these transmittances as well as the durability of the films were still limited for practical use The aim of this work is to improve electrochromic properties of WO3 thin films deposited by electrochemical method The morphology, electrochemical and optical properties concerning with electrochromic performance of the films are also discussed

In addition, we tried to design a new device based on nanostructured MoO3 thin film and poly-(N-vinyl carbazole) according to typical OLED sandwich structure The enhanced photoluminescent performance was investigated and I-V characteristics was also studied

Chapter 1 Overview of transition metal (W, Mo) oxides and

their electrochromic properties 1.1 Introduction of transition metal

In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:

 It commonly refers to any element in the d-block of the periodic table, including zinc and scandium This corresponds exactly to periodic table groups 3 to 12 inclusive

 More strictly, it can refer to those elements which form at least one ion with a partially filled d shell of electrons This is exactly the d-block with zinc and scandium excluded

The first has the attraction of apparent simplicity and is the traditional usage However, many interesting properties of the transition elements as a group are the result of their

ability to contribute valence electrons from s orbitals before d orbitals, a property

which all members of the d-block except zinc and scandium share, so the more

restricted definition is in many contexts the more useful The d orbitals are contributed after the s orbitals because once the d orbital begins to fill its electrons move closer to the nucleus, leaving the s electrons as the outermost

The 40 transition metals: The (loosely defined) transition metals are the forty

chemical elements 21 to 30, 39 to 48, 71 to 80, and 103 to 112 The name transition comes from their position in the periodic table of elements In each of the four periods

in which they occur, these elements represent the successive addition of electrons to

the d atomic orbitals of the atoms In this way, the transition metals represent the

transition between group 2 elements and group 13 elements

Table 1.1 The periodic table of the 40 transition metals

Group 3 (III

B)

4 (IV B)

5 (V B)

6 (VI B)

7 (VII B)

8 (VIII B)

9 (VIII B)

10 (VIII B)

11 (I B)

12 (II B)

Period

4 Sc 21 Ti 22 V 23 Cr 24 Mn 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 Period

Variable oxidation states: The transition metals show a wide variety of oxidation

states because their partially filled d orbitals can accept or donate electrons in chemical reactions A transition element like tungsten or molybdenum has roughly linear

increasing ionisation enthalpies throughout its s and d orbitals, due to the close energy

difference between the 5d and 6s (W) or 4d and 5s (Mo) orbitals Transition metal ions are therefore commonly found in very high states The oxidation states found in compounds of W and Mo are changed from 2 to 6

Properties with respect to the stability of oxidation states:

 Higher oxidation state ions become less stable across the period

 Ions in higher oxidation states tend to make good oxidizing agents, whereas

 The 3+ ions start stable and become more oxidizing across the period

1.2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4]

1.2.1 Crystal structures of tungsten oxide and molybdenum oxide

For Mo oxide, just as for W oxide, the basic structural element is an octahedron with a metal atom at the center and oxygen atoms at the corners, Figure 1.1 Deviations from the ideal cubic perovskite-like structure correspond to antiferroelectric displacements

of W atoms and to mutual rotations of oxygen octahedra The magnitude of the distortion depends on the temperature, which is in agreement with the behavior of most perovskites, and pure WO3 single crystals go through structural transformations according to the sequence tetragonal → orthorhombic → monoclinic → triclinic → monoclinic as the temperature is lowered from 900 to -189oC Tungsten oxide has a tendency to form substoichiometric phases containing edge-sharing octahedra

Figure 1.1 Schematic illustrating a corner-sharing arrangement of octahedra

in a W oxide or Mo oxide crystal

The crystal structure has been studied by high-resolution electron microscopy, and extended defects characterized by crystallographic shear planes, pentagonal

(W, Mo) atoms Oxygen atoms

bipyramidal columns, and hexagonal tunnels have been identified Figure 1.2 demonstrates arrangements of WO6 octahedra surround large defects with hexagonal and pentagonal cross-sections

Figure 1.2 Interpretation of high-resolution transmission electron micrographs

for two crystals of WO 3-z with different stoichiometry

Hexagonal WO3 phases are of particular relevance to electrochromism, as will be mentioned later Hexagonal phases are characterized by a one-dimensional tunnel structure extending through the material An even more open pyrochlore structure of

WO3, with a three-dimensional tunnel structure, was discovered recently It contains some W and O vacancies as well as H3O+ for charge neutrality

1.2.2 Crystal structures of (W, Mo) bronzes and ion intercalated (W, Mo) oxide

Tungsten bronzes can be represented as MxWO3 with M being an atom from the first column in the Periodic Table Their crystal structure depends on the type and density of

range that is displaced towards increased x value for increased ionic radii Such a structure does not exist in pure WO3 but it is possible to extrapolate a lattice parameter for a hypothetical material Tetragonal phases are found at low to intermediate x values for LixWO3 and NaxWO3 and at intermediate x values for KxWO3 Hexagonal phases occur for small incorporation of large ions: KxWO3, CsxWO3, InxWO3 and LixWO3 In case of HxWO3, the hydrogens are thought to be statistically attached to the oxygens as hydroxyl groups, so the material may be adequately represented as WO3-x(OH)x There are reports about an orthorhombic phase at x = 0.1, tetragonal phases for x = 0.23 and x

= 0.33, and a cubic phase for x = 0.5 Modifications of the crystalline structure during

Li+ intercalation/extraction are of particular concern for electrochromic devices

Figure 1.3 Tungsten trioxide crystalline structure with ion M + (H + , Li + , Na + ) intercalation have perovskite-like atomic configurations based on corner-sharing WO 6 ,MoO 6 octahedra

It seen from Figure 1.3 that ion M+ intercalation makes the sample transform according

to monoclinic → tetragonal → cubic with intermediate mixed phases The WO6octahedra are shown as well as the sites available for ion intercalation From an

- M

- W, Mo

- O

inspection of the structures, it is reasonable to expect that only small ions (H+, Li+,

Na+) can be accommodated in the cubic configuration

The open crystal structures of Mo oxide and its hydrates make these materials excellent

as intercalation hosts for H+, Li+, and other ions K0.3MoO3 can serve as a host for cyclic Li intercalation/deintercalation It is possible to prepare LixMoO2 and NaxMoO2with x up to ~ 1 The materials can serve as intercalation hosts and are of interest in battery technology

1.3 Properties of tungsten oxide and molybdenum oxide

Molybdenum oxide films show pronounced electrochromism and have many properties

in similar with tungsten oxide The discussion below covers the optical properties, the electrical properties and electrochromism of these oxide films in common

1.3.1 Optical properties [4, 14]

WO3 crystals have an average refractive index for white light of 2.5 Color changes appear in WO3-z when z is increased, as investigated by Glemser and Sauer Intercalation of alkali ions, so that tungsten oxide bronzes are created, also leads to the development of colors The colors are indicative of a strongly wavelength dependent reflectance Diffuse spectral reflectance of NaxWO3 in the luminous and near-infrared spectral range was reported by Brown and Banks with a reflectance maximum at ~ 0.5

μm for x < 0.2 and a reflectance minimum at ~ 0.5 to ~ 0.7 μm for x ≥ 0.2 In the latter samples, there is high reflectance beyond a certain wavelength that shifts towards smaller values as the Na content is increased At 0.2 < x < 0.5, the reflectance lies primarily in the infrared range, and the moderately high reflectance of blue light

In addition, Faughnan et al studied the optical absorption of an evaporated WO3 film, and proposed a coloration mechanism as follows [31]:

Men+(B) + Men+(A) + e-  Men+(B) + Me(n-1)+(A) (1.1)

Me(n-1)+(A) + Men+(B) + h Men+(A) + Me(n-1)+(B) (1.2) And the polaron absorption spectrum is given by:

(i) Andreson localization in the conduction band as a consequence of strong scattering from the intercalated ions,

(ii) formation of an impurity band and localization due to disorder for x > 0.2, and

(iii) splitting-off of an impurity band as a consequence of electron correlation and Anderson localization in a pseudogap

However recent high-resolution electron spectroscopic data by Hill and Egdell and Hollinger et al showed that the metal-nometal transition at x ≈ 0.2 was due to localization in a pseudogap between an impurity band and the WO3 conduction band, which is in agreement with mechanism (iii).This contention is supported by a numerical analysis by Koslowski and Von Niessen The nature of the pseudogap has been discussed; specifically Hollinger et al suggested a Hubbard gap due to long-range electron interaction, whereas Davies and Franz brought attention to the possible

manifestations of a Coulomb gap due to long-range electron interaction Hill and Egdell argued in favor of localized small polarons

1.3.3 Electro-optical transformation processes (Electrochromism)

The term "electrochromism" was introduced by Platt [4] to represent dependent changes in optical absorption spectra of organic dye molecules dissolved in organic solvents Then, electrochromic materials have been a subject of wide interest with the respect to both fundamental studies of the physical phenomenon itself and their possible application in solar energy management, display devices etc

electric-field- An electrochromic material is able to change its optical properties when a voltage is applied across it The optical properties should be reversible, i.e., the original state should be recoverable if the polarity of the voltage is changed

 Electrochromism is a phenomenon in which the color of a material changes on application of a voltage Electrochromism is well-known in numerous in organic and inorganic substances

Table 1.2 summarizes a number of key properties of the electrochromic oxides The first column indicates the nominal composition of the oxide The second and the third column in the table list the overall optical properties Only some of the oxides mentioned above can be fully transparent to visible light, notably the oxides based on

Ti, Ni, Nb, Mo, Ta, W and Ir Essentially all of the electrochromic oxides are constructed from one type of building blocks, MeO6 octahedra with a central transition metal (Me) atom surrounded by six oxygen atoms It was stressed that the electrochromic oxide films must be permeable to ions and must show some electrical

Table 1.2 Summary of key features for the main electrochromic oxides,

showing oxide type, whether the coloration is cathodic C or anodic A,

whether full transparency can be achieved (yes = Y; no = N) [4]

changes to perovskite The electric charge of tungsten is changed from 6+ to 5+ The

color of tungsten oxide then changes from transparent to blue When the injected electrons and ions are removed from WO3, the color of WO3 changes from blue to transparent

1.4 Applications for electrochromic materials

1.4.1 Electrochromic devices [1, 3]

Figure 1.4 A prototype electrochromic device

Electrochromism, the reversible change in optical properties when a material is electrochemically oxidized or reduced, has a long history of fundamental and practical interest A five-layer prototype electrochromic device, see Figure 1.4, introduces basic design concepts and types of materials In principle an electrochromic device consists

of a transparent electrode of transition metal oxide (WO3, MoO3 thin film) deposited

on a conductive glass (ITO, ATO,…), ion conductor either a thin film or a polymeric

different applications of electrochromic devices are shown in Figure 1.5 [6] Arrows indicate incoming and outgoing electromagnetic radiation and the thickness of the arrow signifies radiation intensity

Figure 1.5 The principles of four different applications of electrochromic devices

1.4.2 Smart window [5]

Electrochromic windows have fueled much of the recent interest in electrochromism Such windows promise actively controlled, continuously tunable light transmission Computer modeling of energy flows in buildings equipped with windows having this capability show significant energy savings in some locations and seasons Then it is possible, with the new technology for 'smart windows ' and 'intelligent' glasses to let the window have variable transmittance It is possible to stop at any intermediate transmittance level, i.e., the films have a „memory‟ so that the voltage needs to be

applied only for altering the optical properties The time for going between bleached and colored state depends on the size of the window, and anything between ten seconds and a few minutes can be regarded as typical These times can be compared with the time it takes for the eye to accommodate

Figure 1.6 Design of smart window

The general reaction in electrochromic windows may be written as [5]:

ECn + yCE m ↔ ECn-a + yCE m+(a/y) (1.4)

state, while y provides for the fact that EC and CE can be present in any stoichiometric

ratio Thin film electrodes require insertion/extraction of ions during redox cycling

Figure 1.7 The transmittance change between coloured and bleached state

of the electrochromic windows in comparison with the eye sensitivity

Figure 1.8 (a) Bleached state and (b) Coloured state of the electrochromic windows

The major function of electrochromic windows so-called 'smart windows' is to control the flow of light and heat passing the building glazing and the glazing of vehicles In future, the electrochromic windows will be massively applied for energy saving in buildings and cars in order to prevent the indoor heating by sun

Figure 1.9 Electrochromic systems using in car

1.4.3 Gas sensors [27]

The regulation, measurement and control of noxious gases in the atmosphere play important role to the environmental protection of most countries It is well known that nitrogen oxide is one of the most polluting and toxic components Its detection is therefore of interest in air quality control, combustion processes and engine emissions

In the latter two cases nitrogen monoxide is then oxidized in the atmosphere to nitrogen dioxide; this latter in turn being converted into organic nitrates and nitrites and presumably into nitric acid by means of chemical and photochemical reactions in the atmosphere polluted by organic substances

According to the work [27], WO3-based sensors exhibit high sensitivity values and are also able to detect very low concentration of NO The detection is carried out by measurements of electrical conductance changes due to gases chemisorbed onto the material surface To prepare WO3-based sensors, tungsten oxide were deposited onto 1-

mm rough alumina substrates 5 x 7 mm2 A four-point Van der Pauw method was used

to measure the electrical properties of the films The sensitivity to NO oxidizing gas

was defined as S=(R g -R a )/ Ra, where Ra is the electrical resistance in air and Rg is the

electrical resistance after exposure to NO

Usually, in stationary conditions nitrogen oxide acts as oxidizing gas towards the most

of n-type metal oxide semiconductors Therefore, as expected, it was observed that the injection of nitrogen oxide leads to a drastic increase in resistivity The electrical resistance variations of the same film exposed to different step of NO concentrations in dry air is reported in Figure 1.11 Also in this case, the resistance of the film reached its starting value after the test gas is shut off and this proves that the absorption process is reversible Furthermore, the advantage in using WO3 sensors is not only applied for detection in the range of ppm concentration but also related to the fast response and recovery times

Figure 1.11 Response of a WO 3 sample at increasing steps of NO concentration

According to the interaction mechanism of the NO molecules with the surface of the material, as in the most of metal oxide sensors, the NO gas in presence of oxygen of the air tends to oxides in NO2 This NO2 can be adsorbed or can interact with the oxygen adsorbed onto the sensors surface according to the following reactions:

NO2g + O2- + 2e- → NO2- + 2O- (1.6) Simultaneously, the NO gas that has not reacted with the oxygen of the air can be adsorbed on the oxide surface and reacts with the oxygen adsorbed In this case the involved reaction will be:

1.4.4 Thin film batteries [17, 33]

The similar features of electrochromic devices and rechargeable thin film batteries concern the material properties, structure of the layers, kinetics of switching (charge/discharge cycles), energy and power density and electrical parameters but also differences between them will be comprehensively discussed in [17] As discussed, one

of the most important type of electrochromic devices is a battery-like window, essentially a transparent rechargeable thin film battery (rocking-chair type) consisting

of a pair of complementary intercalation layers, separated by an ion-conducting polymer electrolyte and contacted by transparent conducting oxide electrodes on glass

At least one of those intercalation electrodes must show coloration by anodic oxidation

or cathodic reduction The battery-like electrochromic device is shown in Figure 1.12

Figure 1.12 Electrochemical reactions, electron and ion exchange processes in an

electrochromic device (TCO: transparent conducting electrodes, IC: ion conducting polymer

film with KA: dissolved salt, EC1 and EC2: complementary intercalation layers,

one or both have to be electrochromic layers, CU: control unit)

The similar features of electrochromic devices and rechargeable thin film batteries concern the material properties (intercalation capability), structure of the layers, kinetics and mechanism of switching (charge/discharge cycles), energy and power

density, extended open circuit memory and other electrochemical parameters, for example, a high exchange current density High electron transfer rates between collector electrodes and active layers, a good electron and ion conductivity within in the active layers and high ion conductivity of the polymer electrolyte present much analogous features too

Essential differences between electrochromic devices and thin film batteries concern the charge density in relation to the efficiency, the open circuit voltage VO, the requirements in regard to porosity, the cycling life time, the discharge depth and the electrical and optical properties of collector electrodes and active layers A viable electrochromic smart window must for example exhibit a cycling life time >105 cycles corresponding to an operation life at 10–20 years In comparison, the cycle lifetime of the batteries is often < 1000 cycles A stringent requirement concerns the charge and discharge depth of electrochromic devices These must be very deep to achieve a high range for the change of optical density, a high transmittance in the bleached state and low transmittance in the darkened state For batteries, an overload and a deep discharge

is bad and must be avoided, since that often leads to the decrease of cycle life and irreversible changes of the structure of the host substances Compared to batteries, the requirements of charge densities for electrochromic windows are quite severe, too A high coloration efficiency and a high speed of the switching cycles require a low charge density for a high response in the optical density Batteries must show a high charge density On the other hand, the electrochromic devices must exhibit a low open circuit voltage (VO < 1.2 V) but VO should be >2 V for batteries There are the weak points of the electrochromic devices that should be overcome in the future

Chapter 2 Photoluminescent properties of nanocomposite materials

2.1 Fluorescence and phosphorescence (photoluminescence) [20]

Luminescence is the emission of light by a substance It occurs when an electron

returns to the electronic ground state from an excited state and loses its excess energy

as a photon Light is directed onto a sample, where it is absorbed and imparts excess

energy into the material in a process called photo-excitation One way this excess

energy can be dissipated by the sample is through the emission of light, or

luminescence In the case of photo-excitation, this luminescence is called

photoluminescence (PL)

 Fluorescence and phosphorescence (Photoluminescence):

Photoluminescence (PL) is the spontaneous emission of light from a material under

optical excitation The excitation energy and intensity are chosen to probe different

regions and excitation concentrations in the sample Photo-excitation causes electrons

within the material to move into permissible excited states When these electrons return

to their equilibrium states, the excess energy is released and may include the emission

of light (a radiative process) or may not (a nonradiative process) The energy of the

emitted light (photoluminescence) relates to the difference in energy levels between the

two electron states involved in the transition between the excited state and the

equilibrium state The quantity of the emitted light is related to the relative contribution

of the radiative process

Photoluminescence investigations can be used to characterize a variety of material

parameters PL spectroscopy provides electrical characterization, and it is a selective

and extremely sensitive probe of discrete electronic states Features of the emission

spectrum can be used to identify surface, interface, and impurity levels and to gauge

alloy disorder and interface roughness The intensity of the PL signal provides information on the quality of surfaces and interfaces

The electronic states of most organic molecules can be divided into singlet states and

triplet states;

Singlet state: All electrons in the

molecule are spin-paired

Triplet state: One set of electron

spins is unpaired

 Fluorescence:

Absorption of UV radiation by a molecule excites it from a vibrational level in the electronic ground state to one of the many vibrational levels in the electronic excited state This excited state is usually the first excited singlet state A molecule in a high vibrational level of the excited state will quickly fall to the lowest vibrational level of this state by losing energy to other molecules through collision The molecule will also partition the excess energy to other possible modes of vibration and rotation Fluorescence occurs when the molecule returns to the electronic ground state, from the excited singlet state, by emission of a photon If a molecule which absorbs UV radiation does not fluoresce it means that it must have lost its energy some other way These processes are called radiationless transfer of energy

Have a look at the following diagram:

Figure 2.1 Possible physical process following absorption of a photon by a molecule

 Intra-molecular redistribution of energy between possible electronic and

vibrational states:

The molecule returns to the electronic ground state.The excess energy is converted to vibrational energy (internal conversion), and so the molecule is placed in an extremely high vibrational level of the electronic ground state This excess vibrational energy is lost by collision with other molecules (vibrational relaxation) The conversion of electronic energy to vibrational energy is helped if the molecule is "loose and floppy", because it can reorient itself in ways which aid the internal transfer of energy

 A combination of intra- and inter-molecular energy redistribution:

The spin of an excited electron can be reversed, leaving the molecule in an excited triplet state; this is called intersystem crossing The triplet state is of a lower electronic energy than the excited singlet state The probability of this happening is increased if the vibrational levels of these two states overlap For example, the lowest singlet vibrational level can overlap one of the higher vibrational levels of the triplet state A molecule in a high vibrational level of the excited triplet state can lose energy in

collision with solvent molecules, leaving it at the lowest vibrational level of the triplet state It can then undergo a second intersystem crossing to a high vibrational level of the electronic ground state Finally, the molecule returns to the lowest vibrational level

of the electronic ground state by vibrational relaxation

 Phosphorescence:

A molecule in the excited triplet state may not always use intersystem crossing to return to the ground state It could lose energy by emission of a photon A triplet/singlet transition is much less probable than a singlet/singlet transition The lifetime of the excited triplet state can be up to 10 seconds, in comparison with 10-5 s to

10-8 s average lifetime of an excited singlet state Emission from triplet/singlet transitions can continue after initial irradiation Internal conversion and other radiationless transfers of energy compete so successfully with phosphorescence that it

is usually seen only at low temperatures or in highly viscous media

2.2 Physics of nanostructured materials [7]

2.2.1 Quantum size effect

The availability of small solids like transition metal clusters offers a filed of research that raises basic questions: How small a number of atoms is necessary until the properties of original metal are lost? How does an ordered accumulation of atoms behave, when it is no longer under the influence of its ambient bulk matter? And perhaps most important for "nanoscientists": What are the future directions for nanotechniques and the applications of the new materials, for example, in microelectronic devices? An increasing number of scientific groups is engaged in

Metal/colloid (a)

Cluster (b)

Molecule (c)

Figure 2.2 Illustration of (a) bulk metal or colloid with typical band structure, (b) a large

metal cluster, and (c) a triatomic cluster with bonding and antibonding orbitals

If a metal particle, initially having bulk properties, is reduced in size to a few hundred

or dozen of atom, the density of states in the valence and conductivity bands decreases

to such and extent (Fig.2.2.a-c) that the electroic properties change dramatically; that

is, conductivity, magnetism, and so on begin to disappear The quasicontinuous density

of states is replaced by a discrete energy level structure, with a level spacing, for example, larger than the characteristic thermal energy kBT (Fig.2.2.b) The situation in molecular clusters is simple Three metal atoms, for instance, form energetically well- defined bonding and antibonding molecular orbitals (Fig.2.2.c) With respect to the size-dependent decreasing electrical conductivity, in 1986 Nimtz et al spoke of the so-called size-induced metal-insulator transition (SIMIT) This refers to the transition caused by a geometrical limitation of extended states with a corresponding De Broglie wavelength when the volume of metallic particles is strongly reduced by fractionation, beginning with a diameter of 1 µm, down to an experimental limit of less than 20 nm This effect was detected by measuring the microwave absorption of small particles of solid or liquid indium metal dispersed in oil Its explanation claims that the boundaries

of these particles are potential walls between which the electrons are localized, and that the conduction electrons form standing waves with multiples of half of the De Broglie wavelength λ/2, thus leading to the picture of "particles in a box" But the corresponding level spacings of these electron states in the preceding size regime are still smaller than kBT, so that the temperature dependence of the conductivity of these particles is very weak This means that the particles are still metallic, but with a size-determined limited number of electrons

This "size quantization" effect may be the onset of the metal - semiconductor transition

at the very end of metallic behavior of much smaller particles The size at which the transition occurs depends on the type of metal and criterion chosen for metallicity as well as the method of investigation Keeping this mind, the term SIMIT should not be fixed to the changer in the electrical conductivity, but generalized by means of additional indicators Indeed, the results of a number of recent physical investigations

on ligand - stabilized transition metal clusters, which a metal nanoparticles consisting

of a defined number of atoms surrounded by a dielectric ligand shell, lead to the conclusion that, coming form the molecular state, metal particles with about 50 - 100 metal atoms and a diameter between 1 and 2 nm are just barely showing metallic behavior

Why are these metallic nanoparticles expected to reveal new electronic properties as soon as the boundary conditions given by their dimensions (e.g.,diameter) reach the order of magnitude of the De Broglie wavelength of the "valency" electrons? The answer is given by quantum mechanics with Heisenberg's uncertainty relation The position and momentum of un electron cannot be determined with the same accuracy The more one electron is spatially confined, the broader its range of momentum Hence, its average energy will no longer be determined by its chemical origin but only

by the dimension Electrons localized as "particles in a box" within zero- dimensional quantum dots lose their freedom in all three dimensions, leading to discrete energy states, as long as their energy is not great enough to break out of this confinement Whenever it is in the magnitude of electron wavelength λ/2 or even smaller, quantum effects govern the wave propagation of the system This effect is called the quantum size effect (QSE) So, by nature, metal clusters of the preceding diameter seem to be chemically size tailored quantum dots in which some few electrons can be localized, both in terms of geometry and quantum mechanics

2.2.2 Quantum Confinement: Superlattices and Quantum Wells

Superlattices and quantum wells were introduced as man-made quantum structures to engineer the quantum states for electrical and optical applications In twenty-five or more years, thousands of papers have been published By 1997, more than 465 patents had been awarded on topics relating to the application of microelectronic and optoelectronic devices, and techniques for producing superlattice materials To realize quantum states in a given geometry, the size must be smaller or comparable to the coherence length of electrons, in order to exhibit quantum interference This requirement eliminates doping as an effective means to achieve confinement, except at low temperatures, because doping comes from charge separation which results in barriers generally far exceeding the coherence length of electrons at room temperatures On the other hand, band-edge alignment of a heterojunction provides abrupt barrier height This short range potential is the consequence of higher orders multiples in the atomic potentials A new type of superlattice was proposed, the Epilayer Doping Superlattice (EDS), consisting of, for example, a couple of layers of

Si in AlP The idea is fundamentally different from atomic plane-doped or δ-doped superlattices where only a small fraction of a plane is occupied by doping or substitution Another type of superlattice designed to incorporate extremely localized interaction, most promising for silicon, was introduced in 1993, consisting of an effective barrier for silicon, formed by a suboxide with a couple of monolayers of oxygen atoms This system, as a barrier for silicon, has been experimentally realized Localized interaction in a man-made quantum system is not new; for example, resonant tunneling involving localized defects was reported Recently, superlattices with

2.2.3 Doping of a Nanoparticle

The doping of a quantum dot is an important issue Since electrochemically etched porous silicon exhibits quantum confinement in photoluminescence, the quantum size effect on doping, including interactions with induced charges at the dielectric discontinuity, requires investigation

Fundamentally, quantum confinement pushes up the allowed energies resulting in an

increase in the binding energy, E b, of shallow impurities such as the cases of quantum wells and superlattices Theoretical treatments of the dielectric constant in quantum confined systems show that a significant reduction takes place when the width of the quantum well is below 2 nm Qualitatively, quantum confinement cuts down the motion of electrons, resulting in a reduction in screening Using the Bohr model for shallow dopants, the binding energy is inversely proportional to the square of the dielectric constant, a reduction in the static dielectric constant greatly increases the binding energy to the extent that most nanoparticles show no extrinsic doping In a

quantum dot of radius a, the measured ε(a) for porous silicon was in fair agreement with the calculation given by Eq (1) the size-dependent dielectric constant ε(a)

ε(a) = 1 + ( ε b - 1 )/[ 1 + ( ΔE/E g)2] (1) However, preliminary calculated binding energy for dopants points out that this reduction in ε plays a small role in the final results because the larger increase is due

to the induced polarization charges at the boundary of the dielectric discontinuity With

ε1 and ε2 denoting the dielectric constants of the particle and the matrix, for ε1 > ε2, the induced charge of the donor is of the same sign resulting in an attractive interaction with the electron of the dot, pushing deeper the ground state energy of the donor

resulting in an appreciable increase in E b For ε2 < ε2, the opposite is true; E b is much

reduced allowing possible extrinsic conductivity at room temperatures

Figure 2.4 Donor binding energy vs dot radius in angstroms for several values

of the dielectric constant of the matrix

Figure 2.4 shows the donor binding energy versus several values of the dielectric constant for the matrix, 1 for air or vacuum, 6 for water within the Helmholtz layer,

etc Note that at a dot radius of 2 nm, the former gives E b = 0.8 eV, while the latter

gives 0.2 eV, making it possible to show extrinsic doping at room temperatures The lack of extrinsic doping as the particle size is reduced by electrochemical etching serves as a limiting factor on the size reduction in etching in the dark If etching is performed with the presence of light, electron-hole generation can lead to the continuous etching without limitation In electroluminescent (EL) diodes, it should be important to match the dielectric constants to facilitate extrinsic doping In fact, this

Extrinsic doping forms the backbone of all solid state devices with pn-junctions It has shown that almost all shallow levels become deep in nanoscale particles, and induced charges at the dielectric interface between the quantum dot and its matrix affect, to a large degree, the binding energy of the dopants Therefore, nanostructured materials in optoelectronic applications require more thought

2.2.4 Excitonic Binding and Recombination Energies

Electrochemically etched porous silicon displays visible luminescence The role of quantum confinement in the porous silicon luminescence is established by the increase

of the optical absorption gap and by the decrease of the Raman phonon frequency with the increase of the peak luminescence energy The quantum confinement effects in silicon nanocrystallites have been treated by tight-binding, effective mass, pseudopotential and first-principles local density approximations However, in order to take into account the induced electrostatic polarization due to dielectric mismatch at the silicon crystallite boundary with the external medium, recombination and binding energies of excitons in silicon quantum dots may be calculated within essentially the same framework as the calculation of the binding energy of a quantum dot The peak recombination energy in silicon quantum dots is quite insensitive to the nature of the external medium due to approximate cancellation of the polarization terms in the recombination energy of the excitons involved No such cancellation is present for the binding energy of the excitons Excitons in silicon crystallites surrounded by vacuum are electrostatically bound by about 1 eV When immersed in water, the binding energy

is dramatically decreased compared to vacuum as the doping case considered in the last section Recombination and binding energies of the excitons confined in silicon nanocrystallites are calculated within the effective mass approximation This approximation has been already applied to evaluate the one- and two-electron ground state energies, donor binding energy, excitonic energy and absorption coefficient The

envelop wave functions of both electron and hole are determined by the kinetic energy

of each particle, which dominates the properties of excitons in quantum dots of the size

in the range of several nanometers The electrostatic terms, Coulomb interaction, polarization interaction, and electron-hole polarization self-energies are treated by perturbation, in a variational calculation As before, the binding energy is defined as the minimum energy required to break an exciton by removing both electron and hole from the same silicon quantum dot and placing them in separate silicon nanocrystals

Figure 2.5 Exciton binding energy, solid, and recombination energy, dashed,

vs dot radius in nm for several values of the dielectric constants of the matrix

Figure 2.5 shows the calculated values of the exciton binding energy, solid; and the recombination energy, dashed; vs dot radius for various ε values of the matrix Note

order of magnitude For the case of the same dielectric constants of the dot and the matrix, where only the Coulomb interaction is present, the exciton binding energy is 0.16 eV, over ten times higher than the bulk value of 14.7 meV This major increase of the Coulomb interaction part of the exciton binding energy is caused by the increased overlap of the electron-hole wave functions For all four values of the dielectric constant of the external matrix, the exciton binding energy is much greater than the characteristic thermal energy at room temperature; therefore, excitons confined to a quantum dot are well bound and stable irrespective of the surrounding matrix Since the self polarization and the polarization terms are very large, any theory not including the dielectric mismatch between the dot and the environment cannot be taken seriously Excitons simply cannot be broken in the usual sense! The activation energy of the break-up of excitons, estimated from the slope of the luminescence decay in air with increasing temperature, is approximately 100–120 meV, which is far less than the exciton electrostatic binding energy (1 eV in vacuum), and is thus related to the turning-on of some non-radiative recombination channels

The photoluminescence in porous silicon at room temperature is due to recombination

of excitons confined in silicon nanocrystals whose effective diameters are approximately 3 nm for nanowires and 3 nm for nanodots The transition is still phonon assisted as in the bulk that involves an electron from the bottom of the conduction band and a hole from the top of the valence band, thus separated in the momentum space In

a well passivated system without surface trap states, a non radiative channel may involve electron tunneling out of the quantum dot It is instructive to compare exciton radiative recombination in silicon nanocrystallites and in bulk Si, both at room temperature In the bulk, it is far likelier that the exciton will be broken by a phonon than to encounter the right phonon for phonon assistance The exciton break up is facilitated by the quasi-continuum of available states in both valence and conduction bands The electron and hole liberated from the exciton by a phonon, fly apart, thus

disabling radiative recombination The quasi-continuum of valence and conduction band states is modified into a discrete set of energy levels due to quantum confinement Since the thermal phonons, without sufficient energy to break up these excitons, allow excitons enough time to wait for the right phonon with the necessary momentum, phonon-assisted radiative recombination occurs In short, it was assumed that nanoscale particles allowed the relaxation of momentum conservation, or even suggested that the band structures may be made direct by nanostructuring It is well known that short of nanostructuring to a dimension of well under 1 nm, indirect band structure still dominates optical transitions The apparent increase in the observed luminescent efficiency is due to the long-lived excitons due to quantum confinement The direct gap in a silicon nanocrystallite is located at 2.9 eV Therefore, some of the observed weak and fast blue luminescence in non-oxidized Si may be from this

component

2.3 Enhance photoluminescent performance of nano-composite materials

2.3.1 Photoluminescence in nc-Si/SiO 2 superlattices

To overcome the problem of structural robustness associated with the porous silicon, it was proposed that nanoparticles of silicon with sizes in the range of several nanometers sandwiched between thin oxide layers to form a superlattice may solve the problem of mechanical robustness while retaining the features of quantum confinement as in porous silicon The name IAG-superlattice was introduced, for Interface Adsorbed Gas superlattice In this scheme, silicon up to 12 nm thick is deposited either in the amorphous phase or crystalline phase, followed by the in-situ growth of a thin oxide

regions with high oxygen content Moreover, the 2.34 eV peak is attributed to surface effects This brings up an important point in all nanostructured materials

Figure 2.6 PL intensity vs photon energy for a nine-period Si/IAG superlattice

annealed in oxygen and hydrogen at 850°C

In devices dictated by bulk, surface or interface regions are considered undesirable As the particle size shrinks to nanometer regime, surface or interface regions become significant or even dominate over the “bulk,” thus surface or interface regions are the focus of their considerations The grain size of the silicon nanoparticles was found to

be approximately 3 nm using Raman scattering and checked by TEM The mechanism controlling the grain size is quite involved Basically, unlike the amorphous-crystalline phase transition in bulk, in very thin structures, the phase transition is controlled by proximity effects rather than simple temperature

2.3.2 Luminescence from Clusters

Elemental semiconductors Ge, Si, and C embedded in an SiO2 matrix exhibited fairly strong and stable PL, with peaks ranging from IR to blue Samples for LED were

fabricated consisting of 45 nm thick polycrystalline Si films deposited initially as amorphous films by e-beam evaporation onto 70 nm SiO2 films thermally grown on n+Si substrates, followed by Ge implantation, to create a supersaturated solid solution

of Ge in the SiO2 film with approximately uniform Ge (~5 nm in diameter) concentration of 5% The samples were subsequently annealed at 600°C, 1×10-6

torr, for 40 minutes to induce precipitation The EL spectrum was broad and peaked at 1.2–1.4 eV Samples of silicon clusters can be prepared by sputtering SiO2 onto silicon wafers without additional heating After annealing at 800°C for 20–30 minutes in N2, the typical PL spectra show typical quantum size effect in Figure 2.7(a–c) with increasing Si to oxide ratio

Figure 2.7 PL of silicon clusters in SiO 2 matrix

2.3.3 Enhanced photoluminescence of Ce 3+ induced by an energy transfer from

for potential application in light emitting devices Masayuki et al, Tomokatsu et al and

Bang reported energy transfer between Eu3+ and CdS, SnO2, and ZnO nanoparticles, respectively, confined in calcined SiO2 gels SiO2 has proved to be a good host matrix for rare-earth elements because of its transparency, compositional variety and ease of production

Enhanced luminescence induced by the energy transfer from nanoparticle crystals (CdS, SnO2, and ZnO) has only been extensively investigated for red line emission from Eu3+ in SiO2 matrix The study [24] discussed on enhanced luminescence of blue broadband emission of Ce3+ resulting from an energy transfer from ZnO nanoparticles

in a SiO2:Ce3+ host

Figure 2.8 Photoluminescent emission spectra from (a) ZnO nanoparticles and

(b) SiO 2 :Ce 3+ , and ZnO–SiO 2 :Ce 3+

Figures 2.8 (a) and (b) show the PL emission spectra of dried ZnO nanoparticles, the average ZnO nanoparticle diameter was 4 nm, and dried and calcined SiO2:Ce3+ and ZnO–SiO2:Ce3+ powders, respectively The fact that the wavelength of the main Ce3+emission peak did not shift between SiO2:Ce3+ and ZnO–SiO2:Ce3+ suggests that the incorporation of ZnO nanoparticles did not change the radiative relaxation process for

Ce3+ The data also suggest that the ZnO nanoparticles were well dispersed in the SiO2matrix Note from the data in figure 2.8(b) that green emission from ZnO nanoparticles

in ZnO–SiO2:Ce3+ has been completely quenched and that the intensity at 417 nm from ZnO–SiO2:Ce is 4 × larger than the intensity from SiO2:Ce3+ Quenching of the ZnO emission and enhancement of the Ce3+ emission demonstrates that ZnO nanoparticles absorbed energy from the excitation source (325 nm HeCd laser) and transferred it nonradiatively to luminescent centres (Ce3+ ions)

Figure 2.9 Possible transitions in Ce 3+ and mechanism of energy transfer

from ZnO nanoparticles.

Postulated mechanisms to account for transitions in ZnO nanoparticles and Ce3+, and a subsequent energy transfer between ZnO nanoparticles and Ce3+ ions, are shown in figure 2.9 The bandgap excitation of ZnO has resulted in creation of an exciton, and subsequent nonradiative recombination results in excitation from the ground 4f states

to the excited 5d states on the Ce3+ centre Subsequent radiative relaxation on the Ce3+